This invention relates to electrically driven power tools and, in particular, to a power tool such as a variable speed drill having a motor control that is adapted to increase and better control the effective torque output of the tool.
Electrical power tools, such as variable speed drills and power screwdrivers, typically include a motor control circuit that is adapted to control the speed of the output spindle of the tool by controlling the amount of current supplied to the motor. The desired motor speed is usually selected by the operator by varying the position of the trigger switch.
If the power tool is provided with an open loop motor control circuit, the speed of the output spindle of the tool will decrease as the tool is loaded and the current drawn by the motor will increase. Accordingly, if a relatively constant output speed is desired, the operator must manually compensate for the reduction in motor speed as the tool is loaded by further retracting the trigger switch to increase the power applied to the motor. If the power tool is provided with a closed loop motor control circuit, the control circuit is typically designed to automatically increase the amount of power supplied to the motor as the output spindle of the tool is loaded in order to maintain the desired speed.
Thus, when employed in a power screwdriver to drive a screw into a workpiece, for example, the current drawn by the motor will increase as the torque required to drive the screw increases, regardless of whether the control circuit provides open or closed loop control. This operation will continue until either the operator releases the trigger or the motor stalls as the increased torque required to drive the screw exceeds the torque capacity of the tool. Consequently, the effectiveness of many portable power tools, particularly power screw-drivers, is directly related to the tool""s maximum torque output capacity. Obviously, the greater the output capacity of the tool, the more useful and versatile the tool. However, in order to significantly increase the torque output capacity of a tool, it is generally regarded as being necessary (assuming changes to the gear train are not an option) to increase the size of the motor and, consequently, the size, weight, and cost of the tool.
Accordingly, it is the primary object of the present invention to provide a portable electrical power tool having a motor speed control circuit that is able to substantially increase the effective torque output of a power tool for a given size motor and gear train.
In addition, it is an object of the present invention to provide a portable electric power tool, such as a power screwdriver having a motor control circuit, that enables the operator to better control the torque output of the tool, which is particularly beneficial when driving a screw into a workpiece.
The motor control circuit employed in the present invention is able to achieve these objectives by intermittently pulsing the motor for predetermined periods of time after a threshold current level is attained. More specifically, it has been found that if power to the motor of a power drill is reduced for a length of time sufficient to allow the gear train coupled to the motor to at least partially xe2x80x9crelaxxe2x80x9d, and then power is increased, the motor is able to build up potential energy before the looseness (i.e., backlash) is removed in the gear train. In effect, the motor is afforded a xe2x80x9crunning startxe2x80x9d while the gear train is relaxed. When the backlash in the gear train is removed, the sudden impact of the motor torque on the gear train causes a sudden and high burst of torque to be imparted to the output spindle of the drill, and hence to the driving bit secured thereto. When this pulse control of the motor is repeated, the motor is able to provide a series of bursts of torque to the gear train which in turn can be used to better finish driving a wood screw into and below the surface of a workpiece. The present control scheme thus provides better user control due to the fact that the screw does not rotate too much when static friction is overcome. Rather, with each torque pulse, static friction is overcome and the screw is incremented a fraction of a turn.
While the pulsing operation described above has been found to be particularly helpful and effective when used to drive wood screws and other like implements into a work surface, it has also been found to be an effective means for xe2x80x9cbreaking loosexe2x80x9d a screw or like fastener which is tightly seated in a workpiece, where other forms of power tools such as conventional variable speed drills are unable to do so. By reversing the action of the variable speed drill and applying the pulsing operation described above, the bursts of high torque applied by the motor have been found to be extremely effective in overcoming the high level of stiction force required to initiate removal of such fasteners.
Accordingly, it is a further object of the present invention to provide an electrically driven power tool, such as a variable speed power drill, which incorporates a control circuit for controlling a motor thereof such that the motor can be alternately pulsed fully on and then fully off at a predetermined cycle time during operation of the drill.
It is another object of the present invention to provide an electrically driven power tool having such a control circuit that further provides an operator of the power tool with a means for adjusting the point at which the alternating full-on and full-off operation is initiated.
It is yet another object of the present invention to provide an electrically driven power tool which automatically enters the alternating full-on and full-off mode of operation when the current through the motor exceeds an operator adjustable threshold level setting.
Additionally, it is an alternative object of the present invention to provide an electrically driven power tool that provides the operator with control over the magnitude of the torque bursts during the alternating phase of operation of the tool.
Finally, it is an object of the present invention to provide simplified versions of the present motor control circuit that are suitable for use in relatively low-cost power tools.
The above and other objects are provided by a portable electric power tool having an electronic control circuit and method in accordance with preferred embodiments of the present invention. The control circuit is preferably disposed within the housing or body of the electrically driven power tool, which is represented illustratively herein as a variable speed power drill. The control circuit generally comprises operator adjustable means for setting a threshold current level which defines a transition point at which alternating on and off operation of the motor is initiated; a trigger switch for selecting the desired speed of the motor; a current sensing circuit for sensing the current flowing through the motor; a switching circuit for controlling the flow of current to the motor; and a controller for comparing the current sensed by the current sensing circuit relative to the threshold current level selected by the operator and controlling the switching circuit to control the amount of current applied to the motor. When the current drawn by the motor exceeds the selected current threshold level, the controller is adapted to temporarily interrupt current flow for a predetermined xe2x80x9coff-timexe2x80x9d interval, and then reapply a maximum current signal for a predetermined on-time interval, and to alternate this on and off operation until the trigger switch is released.
The off-time interval during which the controller causes the switching circuit to temporarily interrupt current flow to the motor is sufficient to allow the gear train coupled to the motor of the power tool to sufficiently xe2x80x9crelaxxe2x80x9d before maximum current is reapplied to the motor. A value representing this time duration is preferably stored in a memory of the controller and is unique to the gear train of the particular power tool being controlled.
By alternately applying a maximum current signal for a desired time and then interrupting current flow for a predetermined time, the motor of the power tool is caused to generate successive xe2x80x9cburstsxe2x80x9d of torque to the gear train of the power tool which significantly increases the effective torque output of the power tool. This technique further has been found to be extremely effective in xe2x80x9cbreaking loosexe2x80x9d tightly set wood screws and the like, which other conventionally controlled power tools having comparable-sized motors are unable to achieve.
In several preferred embodiments of the invention, a memory is included for storing a plurality of predetermined xe2x80x9con-timesxe2x80x9d which the controller accesses depending on the setting of the current threshold level setting means. Thus, on-times of varying duration can be selected by the controller to precisely meet the anticipated conditions of a specific application.
In an alternative preferred embodiment of the present invention, the current comparison performed by the controller is modified in accordance with the changing (i.e., increasing) speed of the motor as the trigger switch is squeezed during operation of the power tool to increase motor speed. In this instance the threshold current level signal selected by the operator is decreased as the speed of the motor increases. With this embodiment a speed sensor is employed to monitor the speed of the motor and provide a signal representative thereof to the controller. As the speed of the motor increases due to progressive engagement of a trigger of the power tool, the controller decrements the operator-selected threshold current level signal. This alternative embodiment further helps to compensate for the inertia of the gear train at higher motor speeds and helps provide even more consistent results independent of the motor speed of the power tool.
In yet another alternative preferred method of operation of the present invention, the transition point for beginning the alternating on and off operation (referred to also as the xe2x80x9cratcheting modexe2x80x9d of operation) of the motor is determined in accordance with a predetermined percentage increase in the sensed motor current. With this method the current through the motor is initially measured. After a predetermined time delay, a second current measurement is made. This operation is repeated continuously until the second current measurement exceeds the initial current measurement by a predetermined factor. At that point the controller initiates the alternating on and off operation of the motor. In this embodiment the operator-adjustable threshold current level means is replaced by a means for allowing the operator to adjust the desired on-time of the motor once the ratcheting mode of operation has begun.
This embodiment and method of operation thus provides a method for xe2x80x9cautomaticallyxe2x80x9d sensing the size of a screw (and thus the torque required to drive the screw) as the operator begins driving the screw into a workpiece, based on the initial current reading. Since the current required to drive a large screw is greater, in the initial stage, than that required for a small screw, setting the transition point in accordance with a predetermined increase in current (e.g., 25% or 50%) automatically serves to adjust the transition point at which the ratcheting mode of operation begins in accordance with the size of the screw being driven.
In yet another alternative preferred mode of operation of the present invention, the transition point is determined by a predetermined drop in motor speed. In this embodiment, the ratcheting mode of operation of the motor is initiated when the motor speed drops below a predetermined speed, or by a predetermined amount (i.e., percentage), or by a predetermined rate.
In a further alternative embodiment, it has been determined to be advantageous to provide the operator with control over the magnitude of the torque bursts during the ratcheting mode of operation. In other words, rather than providing fixed on-time/off-time periods during the ratcheting mode of operation, it may also be desirable to provide the operator with the ability to continue to vary the duty cycle of the voltage signal during the ratcheting mode of operation in accordance with the position of the trigger switch.
In particular, conventional variable speed power tools control the speed of the motor by varying the duty cycle of the voltage signal supplied to the motor. The frequency of the duty cycle signal is set sufficiently highxe2x80x94typically 1 KHz to 12 KHzxe2x80x94so that the motor operates smoothly even though the power is actually being rapidly cycled on and off. The percentage on-time of the duty cycle signal, and hence the average power level, supplied to the motor is controlled by the operator in accordance with the position of the trigger switch.
Consequently, it will be appreciated that the transition from normal variable speed control of the motor to the above-described ratcheting mode of operation can be viewed simply as a change in the frequency of the duty cycle control signal. In other words, the ratcheting mode of operation can be achieved simply by switching from a relatively high frequency control signal to a relatively low frequency control signal (e.g., 10-50 Hz), the period of which is greater than the response time of the motor. Considered in this manner, it is readily apparent that it is possible to continue to provide trigger switch control over the duty cycle of the control signal during the ratcheting or low frequency mode of operation, and thereby provide the operator with the ability to control the magnitude of the torque bursts. This, in turn, provides the operator with greater control when seating a screw into a workpiece.
Lastly, various simplified versions of the present invention are disclosed. In these alternative embodiments, the motor control circuit does not automatically transition between conventional variable speed control and the low frequency pulse mode of operation. Consequently, the more sophisticated microcomputer-based controller, as well as the feedback circuitry for sensing motor current and speed, can be eliminated.
In a first version of this simplified form of the present invention, the power tool is continuously operated in the low frequency pulsing mode. In particular, it has been found that even at a low duty cycle frequency, such as 10 to 50 Hz, the speed of the motor can be varied by varying the duty cycle of the control signal to the motor. Moreover, because the off periods are relatively short (typically less than 10 msec.) at high duty cycle settings, the application of torque from the motor to the output spindle of the tool is relatively smooth. However, as the duty cycle signal is reduced, the off periods between successive pulses increase, thereby producing a more pronounced pulsing operation. This form of motor speed control is thus particularly advantageous when controlling a drill to drive screws. In particular, an operator typically operates a conventional variable speed drill at or near full power (100% duty cycle) during the initial stage of driving a screw and then slows the motor (low duty cycle) during the final stage when the head of the screw is being seated to the proper depth. This technique is therefore readily compatible with the described simplified form of motor speed control as the motor operates in a conventional manner at full power with the pulsing action becoming more apparent at lower duty cycle settings. Thus, the operator is provided with significantly improved control over the depth to which the screw is set. Moreover, despite the relatively low duty cycle settings, the pulsing action produces enhanced bursts of torque which drive the screw in controllable incremental amounts, thereby permitting the operator to accurately set the depth of the screw.
In an additional alternative embodiment of the simplified version of the present invention, a selector switch is provided for enabling the operator to switch between a normal high frequency xe2x80x9cdrillingxe2x80x9d mode of operation and a low frequency xe2x80x9cscrew drivingxe2x80x9d mode of operation.
Finally, a further alternative embodiment of the simplified version of the present invention is disclosed that provides a separately adjustable control knob for varying the frequency of the PWM signal. More particularly, the trigger switch in this embodiment functions in a conventional manner to control the duty cycle of the PWM signal. An additional operator actuable knob or dial, preferably located on the top of the drill, is provided for selectively setting the frequency of the PWM signal. Thus, for example, when driving small screws requiring more control, a moderately low frequency (e.g., 50 Hz) can be used which reduces the maximum off time between successive pulses and thereby limits the magnitude of the torque spikes applied to the screw. However, when setting large screws, a lower frequency (e.g., 10 Hz) can be selected which results in sufficiently long off periods to enable the gear train to completely relax, thereby allowing the subsequent build-up of greater potential energy and the application large torque bursts to the screw. Additionally, the operator can simply set the frequency to a normal high PWM frequency level (e.g., 12 KHz) for operation as a conventional variable speed drill.